I can't get a SpaceView3D draw callback to work
All possible callbacks


Prints to console when callbacks fired

So much info in the python console, autocomplete on draw handler add. Will see that args is a tuple of arguments passed to the callback method.  Error in question was passing 1 arg to a method with none.  Will show up in system console as an error.

&gt;&gt;&gt; bpy.types.SpaceView3D.draw_handler_add(
draw_handler_add(callback, args, region_type, draw_type)
.. method:: draw_handler_add(callback, args, region_type, draw_type)
Add a new draw handler to this space type.
It will be called every time the specified region in the space type will be drawn.
Note: All arguments are positional only for now.
:param callback:
   A function that will be called when the region is drawn.
   It gets the specified arguments as input.
:type callback: function
:param args: Arguments that will be passed to the callback.
:type args: tuple
:param region_type: The region type the callback draws in; usually ``WINDOW``. (:class:`bpy.types.Region.type`)
:type region_type: str
:param draw_type: Usually ``POST_PIXEL`` for 2D drawing and ``POST_VIEW`` for 3D drawing. In some cases ``PRE_VIEW`` can be used. ``BACKDROP`` can be used for backdrops in the node editor.
:type draw_type: str
:return: Handler that can be removed later on.
:rtype: object


Attempt to add every draw call back possible.  Has to be a redraw of the type specified to fire the callback.  I doesn't relentlessly fire.

import bpy
from bpy.types import SpaceView3D, Region
class DrawClass:
    def __init__(self, context):
        print("INIT")
        for rt in  Region.bl_rna.properties['type'].enum_items.keys():
            for d in ('POST_PIXEL', 'POST_VIEW', 'PRE_VIEW', 'BACKDROP'):

                try:
                    SpaceView3D.draw_handler_add(
                        self.dcb, (rt, d,), rt, d)
                except:
                    print("No dcb for", rt, d)


    def dcb(self, rt, d):
        print("Testing dcb:", rt, d)


if __name__ == "__main__":
    DrawClass(bpy.context)